Protein Trafficking Flashcards
Which of the following pathways does not utilize vesicular transport
a) Golgi –> ER
b) secretory vesicles –> cell exterior
c) lysosome to late endosome
d) nucleus to cytosol
d) gated transport
What are the two ways that a signal can be built into a protein?
1) signal sequence (continuous linear sequence)
2) signal patch (3D conformation of signal sequence)
What three types of nucleoporins are found in NPC?
TM ring proteins (anchor NPC to envelope) scaffold nucleoporins (form layered ring structures) channel nucleoporins (line central pore)
How do the receptor-like fibrils that protrude from NPC differ?
On nuclear side, converge to form basket like structure
What size of molecules diffuse into nucleus freely via aqueous pores?
5kDa or less
What size proteins cannot enter nucleus via passive diffuse
50kDa and above
Describe the role of Ran in nuclear import
In nucleus, Ran-GTP binds to import receptor and cargo is released.
Once import receptor back in nucleus, Ran-GAP dephosphorylates Ran-GTP and it dissociates.
Import receptor can pick up cargo again.
Describe the role of Ran in nuclear export
Ran-GTP promotes cargo binding to export receptor
What is NFAT?
nuclear factor of activated T-cells
Describe the cycle of NFAT during T cell activation
- T cell activation causes high calcium in active T cell.
- Calcineurin binds and dephosphorylates NFAT, exposing its nuclear import signal.
- NFAT imported, activates IL-2 gene transcription.
- Ca++ eventually decreases
- Calcineurin drops off and NFAT is phosphorylated (using ATP) exposing nuclear export signal
What is the role of IL-2?
Stimulates clonal expansion of memory T cells
How does cyclosporine work to prevent organ rejection?
Inhibits calcineurin, prevents phosphorylation of NFAT and therefore NFAT translocation into T cells
What occurs to collagen polypeptide after translation? (up until ER)
Signal sequence on polypeptide is targeted to ER by signal recognition particle
Describe co-translational translocation
- Signal recognition particle binds to signal sequence, and translation is paused until attaches to SRP receptor in ER membrane
- Translation continues and translocation begins
What does the N-terminal signal sequence also act as?
START-TRANSFER sequence
The start-transfer sequence binds to ___________ and then is discharged into the bilayer once translocation is complete
active translocator
Describe the mechanism by which LDL receptor is placed in plasma membrane (4 steps)
- Start transfer sequence opens translocator
- Stop transfer sequence enters translocator
- Translocator opens and discharges start transfer sequence laterally
(one pass transmembrane protein)
What three changes occur in ER lumen that lead to formation of the triple helix in collagen?
1) signal peptide of N terminus dissolves
2) hydroxylation of lysines and prolines
3) glycosylation (addn of glucose/galactose monomers onto hydroxyl groups) on lysines
Why does lack of Vitamin C cause scurvy?
Because Vitamin C is a cofactor in the hydroxylations of lysines and prolines during the formation of the triple helix of collagen therefore collagen triple helices are loose
Does stop transfer sequence stop translation?
NO
When a single pass transmembrane protein is inserted into the ER membrane, what determine that the N terminus is on the cytosolic side?
If more positively charged AA immediately proceed hydrophobic core of start transfer sequence
How does a multipass transmembrane protein get inserted
same process as any transmembrane protein but translocator does not discharge start transfer sequence laterally until FINAL stop transfer sequence has entered
What is the significance of coated vesicles? Name three types
Different protein coats select different cargo. COPI, COPII, clathrin
After being processed in the golgi apparatus, what happens to procollagen?
It is packaged by COP1 proteins into a secretory vesicle destined for extracellular space
What occurs in the regulated secretory pathway?
Selected proteins in TGN are directed into secretory vesicles where proteins are concentrated and stored until extracellular signal stimulates their secretion
The human pancreas has the largest capacity for protein synthesis of any organ in human body. What is much of capacity used for?
synthesizing digestive enzymes in acinar cells
Which of the following enzymes is not stored in pancreas?
a) alpha amylase
b) beta lactamase
c) lipase
d) RNAse
e) DNAse
b
What is a pancreas consensus element?
They are enhancer regions found on genes for digestive enzymes that regulate transcription of mRNAs
________, a transcription factor, binds to pancreas consensus elements and essential for __________________
PTF-1, expression of digestive enzymes
What is the endocrine function of the pancreas?
Produce and secrete insulin and glucagon
What occurs in exocrine pancreatic insufficiency?
Pancreas is damaged and cannot produce enough enzymes to digest food –> fail to get nutrition from food eaten
Which of the following is NOT a disease caused by exocrine pancreatic insufficiency?
a) chronic pancreatitis
b) celiac disease
c) crohn’s disease
d) diabetes
e) all of the above are caused by EPI
e)
in Zollinger Ellison, what causes EPI?
acid mediated deactivation of pancreatic enzymes
What causes EPI in Celiac disease?
Decreased pancreatic stimulation
Obstruction of the main pancreatic duct can result in?
Tumor
What type of transport do clathrin coats mediate?
receptor-mediated vesicular transport
What do clathrin coated vesicles accomplish in TGN?
1) sort lysosomal proteins for transport to lysosomes
2) sort substrances for storage in secretory vesicles
What do clathirin coated vesicles accomplish at plasma membrane?
Used for uptake of extracellular material via RECEPTOR MEDIATED ENDOCYTOSIS
Digestive enzyme exocytosis is stimulated by what?
neurohormal agents such as acetylcholine
Clathrin coats mediate formation of secretory vesicles also called what?
zymogen granules
When someone eats, the exocrine pancreas must replenish stores of digestive enzymes. This triggers three processes:
1) unfolded protein response - inc synth of chaperones and foldases
2) upregulation of systems involved in degradation of un/mis folded proteins
3) ER-associated protein degradation = required to rid cell of mis/un folded toxic proteins
The adaptive unfolded protein response has 3 functions
1) inc expression and funcn of chaperones and foldases
2) activate ERAD
3) global reduction in translation of mRNA everywhere ELSE
Where does calnexin, the membrane bound chaperone bind to incompletely folded proteins?
1 terminal glucose on N-linked oligosaccharide
Where are proteins glycosylated ? (location of cell)
ER lumen
What happens to protein once it is bound by calnexin?
It is trapped in the ER to refold, terminal glucose is removed in process. After released, glucose transferase determines if folded properly. if not, glucose added to restart process
How are proteins degraded through ERAD?
Proteasome
Liver is central to the regulation of what?
Cholesterol levels - synthesizes for export and removes from body by converting to bile salts
Describe LDL cholesterol
“bad”, main carrier of cholesterol in circulation, main source of build up and blockage in arteries
Describe HDL cholesterol
good, helps remove cholesterol from arteries
Describe triglycerides
Another form of fat in blood that can raise your risk of heart disease
What molecule is recognized by LDL receptor for removal of LDL from circulation and delivery of cholesterol to cell?
apolipoprotein B10
Cholesterol is a precursor for (3)
steroid hormones, bile acids, vitamin D
Receptor mediated endocytosis of LDL
LDL binds to LDL receptor in ER membrane and is endocytosed through clathrin coated pit
What type of mutation can lead to hyperchoelesteremia?
LDL receptor protein with defective coated pit binding site
Familial hypercholesteremia is a ___________ disorder caused by mutations in LDL receptors
autosomal dominant
Heterozygotes for FH are present in 1 / ______ people whereas homozygotes are 1 / _____ people
500, million
In homozygotes for FH, sudden death and acute MI may happen as early as ______ years old
1-2 years
In heterozygotes for FH, CAD in men develops in _____
40s - women delayed by 10-15 yrs
How is free cholesterol released in bloodstream (start in ER)
LDL endocytosed –> vesicle uncoated -> fusion with endosome –> lysosome –> free cholesterol released
Lysosomes contain what type of pump
ATP driven H+
What tag diverts proteins to lysosome
mannose 6 phosphate
What are the three paths towards lysosomal degradation
endocytosis
autophagy
phagocytosis
